Project Summary Bmi1 (B-cell specific Moloney murine virus insertion site 1) gene has been identified as an oncogene in multiple cancers, inducing B- and T-cell leukemias. Bmi1 is a stem cell gene, which determines the proliferative capacity and self-renewal of normal and leukemic stem cells. Multiple studies identified Bmi1 protein to induce oncogenic transformation and promote tumor growth in a variety of in vitro and in vivo models. Therefore, targeting Bmi1 activity represents a very promising approach for development of novel therapeutics and attractive agents to eradicate cancer stem cells. Bmi1 protein is a component of the polycomb suppressive complex 1 (PRC1), and has been shown to play a critical role in cell cycle regulation by acting as a transcriptional repressor of INK4a/ARF locus encoding two important tumor suppressors p16Ink4a and p14Arf. Bmi1 is a relatively small, 37 kDa protein composed of N-terminal RING domain which has E3 ubiquitin ligase activity and the second domain (B2D, the Bmi1 2nd domain) which represents a protein-protein interaction motif. Protein-protein interactions of the B2D domain are essential for Bmi1 activity. It has been shown that B2D is involved in the interactions with polyhomeotic proteins within the PRC1 complex as well as with the zinc-finger transcription factors E4F1, Zfp277 and PLZF- RARA fusion protein. In this project we propose to develop small molecule inhibitors targeting the B2D protein-protein interaction domain of Bmi1 as new chemical probes to further study the role of B2D domain of Bmi1 and to identify new potential anti-cancer agents. We have already characterized the interaction of B2D domain of Bmi1 with PHC2 and developed high quality biochemical assays suitable for screening of small molecule libraries. We will carry out high-throughput screening at the Center for Chemical Genomics (CCG) at the University of Michigan to identify small molecule inhibitors of this protein-protein interaction. We will employ interdisciplinary approach combining biochemical assays, biophysical experiments and structural biology methods to validate HTS hits and identify high quality small molecule inhibitors of Bmi1. Activity of the most potent inhibitors will be evaluated in a panel of cell-based experiments to assess their capability to inhibit the self-renewal properties of leukemic cells. Our project represents a very novel and innovative approach to target Bmi1 through blocking the protein-protein interactions as currently there are no inhibitors of Bmi1. In summary, we expect to identify highly valuable compounds that can serve as chemical probes suitable for mechanistic studies and for further development into potent in vivo inhibitors of the oncogenic Bmi1.